In Kilauea, Hawaii, little stresses add up

The Kilauea volcano in Hawaii is one of the most active in the world. Its 2018 eruption was the largest in 200 years and one of the costliest volcanic disasters in U.S. history. As devastating as it was to the Hawaiian landscape, Kilauea’s eruption proved a boon for science. The researchers were there to collect unprecedented seismic and other data and said we could use it to understand the stresses that lead to major earthquakes.

“Kilauea is a model seismic system,” said Paul Segall, the Cecil H. and Ida M. Green Professor at the Stanford Doerr School of Sustainability and lead author of the new study published in Natural geosciences which leverages this new data to better understand large earthquakes along tectonic fault lines, like the San Andreas and others.

“Kilauea gives us a unique chance to study many earthquakes over a short period of time, which is not possible with tectonic earthquakes with recurrence intervals of hundreds or even thousands of years .”

Every day or so from May to July 2018, the Kilauea caldera – a circular block of solid rock 8 km in diameter and 800 m thick weighing millions of tonnes – collapsed into a pool of molten rock below , sending shockwaves.

Over the decades, scientists have developed extensive seismic and ground deformation monitoring networks in Kilauea. Before the event and at the start of the eruption, researchers supplemented these networks with sensors on and around the summit of the collapsing volcano, providing a scale of observations never before available. They used real-time Global Navigation Satellite System (GNSS) receivers, ground tiltmeters, cameras, drones and other sensors to trace the summit’s step-by-step collapse into the molten magma reservoir below.

For the first time, they were able to estimate the stresses acting on the volcanic system during caldera collapse and identify how these stresses caused a series of smaller shocks, which accelerated into larger, more frequent, ultimately resulting in major collapse earthquakes.

The team focused on the last 29 of the 62 approximately day-long peak collapse cycles that Kilauea experienced during the summer of 2018. Each collapse unfolded through a series of many small shocks but has always ended with earthquakes exceeding magnitude 5 on the Richter scale, which is quite considerable. in seismic terms.

Each collapse cycle began when magma flowed rapidly from beneath Kilauea’s summit to fuel eruptions miles away. The roof of the magma chamber – this huge block of solid rock – would collapse into the molten reservoir, accentuating the annular fault surrounding the caldera. Then, very quickly, it fell – meters at a time – into the reservoir, pressurizing the underlying magma. Then the cycle would start again.

“We had GNSS instruments on the collapsing block of rock, as well as outside the ring fault. As the block fell, these instruments fell as well and we could measure the changes,” said the co-author Kyle Anderson, researcher. geophysicist with the US Geological Survey who was part of the team working on site at Kilauea during the 2018 eruption.

“The subsidence repressurized the magma chamber and pushed the rest of the summit up,” Anderson noted of one of the study’s surprises. “People think of a caldera collapse as a collapse of everything. And that’s true for the block itself. But not for the surrounding rock during these events.”

Recognize patterns

The researchers also noticed another trend. Before a collapse, increasing stresses on the overlying crust caused hundreds of small earthquakes, called pre-quakes. Eventually, large tremors spread all around the 5-mile ring fault and the ice sheet collapsed into the magma reservoir. Importantly, the researchers were able to use the rise and subsidence of the ground outside the caldera to estimate the history of stress changes inside the volcano.

“Over 29 cycles, we can look at this stress variable and compare it to the length of intervals between earthquakes,” said Segall, a geophysics professor. “The faster this stress builds up, the more frequent earthquakes should be, and that’s what the data shows.”

The researchers also found that small earthquakes accelerated in frequency and intensity just before major earthquakes.

“As we got closer to the mainshock, we saw a greater propensity for larger events,” Segall said. “You are getting closer to a state where an earthquake can erupt and grow to a larger size. In the last 10 to 15 minutes, the proportion of larger events has increased much higher than in the last 12 previous hours, as the system approached this breaking point.

Little stresses add up

This threshold phenomenon explains how small increases in stress can lead to dramatic changes in the probability of increased earthquake magnitude. Building on this idea, Segall’s team proposed a model, based on previous theoretical studies, of how bumps and other rough spots on the ring fault cause changes in localized stress levels that trigger and prevent earthquakes.

“We think this roughly circular fault is very irregular,” Segall said. “After a mainshock, when the stress should be low on average, we start to have small earthquakes that rupture for some distance and then move toward an unfavorable part of the fault. When the overall stress level increases “The chances of it popping up. That and the size increases, and you can reach a point where an earthquake goes all the way around the fault and becomes a magnitude 5.3 or 5.4 earthquake.”

This research could be used to understand earthquakes in different environments, such as the San Andreas Fault or the Cascadia Subduction Zone. Segall’s team suggests that the physical roughness of a fault controls the likelihood that an earthquake, once triggered, will develop into a potentially damaging shock. Researchers could attempt to calibrate this behavior by using small tremors to determine the likelihood of a future damaging earthquake in different environments.

“Many of the same physical processes in volcanoes are at work on other faults,” Segall said. “While we’re not going to predict earthquakes based on our research any time soon, we think the lessons learned and overall knowledge about how the system works should be true. The question would be understanding enough of the details to be able to apply them in other settings.”